This invention relates generally to communication networks and more particularly, it relates to a daisy chain collision detection circuit for use with a StarLAN (proposed IEEE 802.3: 1BASE5 Specification) coded data transceiver.
As defined herein, a daisy chain connection, otherwise known as a passive serial bus or multipoint extension (MPE), is used as a means of interconnecting a plurality of data terminal equipments (DTE). The daisy chain connection refers to the joining together of receivers from the plurality of DTE's onto a first pair of wires and the joining together of transmitters from the plurality of DTE's onto a second pair of wires either by immediate connection or by drop cable. The respective receiver and transmitter wire pairs are either connected to a StarLAN hub or, if there is no StarLAN hub, joined to each other at one point only.
There is shown in FIG. 1 a StarLAN (an acronym for Star Local Area Network) daisy chain arrangement in which a plurality of transceivers DTE #1, DTE #2, . . . DTE #n are star-connected (i.e., a star topology network) to a shared hub 10 via two twisted pairs of transmission lines 12 and 14. Each of the transceivers DTE #1, . . . DTE #n consists of respective transmitters TxD1, . . . TxDn and respective receivers RxD1, . . . RxDn. As can be seen, the twisted pair of transmission lines 12 defining a transmit channel is connected to each of the transmitters DTE #1, . . . DTE #n via isolation transformers 16. The twisted pair of transmission lines 14 defining a receive channel is connected to each of the receivers RxD1, . . . RxDn via isolation transformers 18. In operation, when only one of the transmitters is transmitting the hub 10 repeats the transmitted signals and sends them to all of the other transceivers. However, when two or more transmitters are transmitting concurrently referred to as a "collision" it is needed to detect a StarLAN daisy chain collision condition at each active transmitter.
In order to determine the occurrence of such a collission, there has been provided heretofore a collision detector circuit connected between each of the transmitters and the isolation transformers 16 at the place marked by an "X". This prior art collision detection circuit was formed of a peak detector circuit utilizing a fixedd D.C. reference voltage of approximately +5.0 volts .+-.5%. Since the output voltage of the transmitter is susceptible to variations due to changes in the load, the peak detector circuit could be falsely triggered at times to indicate a collision when there was none thereby limiting the reliability and accuracy thereof. Also, sensitivity to collisions was limited due to the need to maintain adequate margins between the reference voltage and the peak voltage under all operating conditions of the transmitters. Thus, there is a need for improvements in such daisy chain collision detection circuits to provide a more accurate referenced voltage which is proportional to the peak output voltage of the transmitter.